watchOS uses SF Compact, part of Apple’s San Fransisco family.
It is not licensed for use outside watchOS & is limited to development for that OS only.
Ref: Wikipedia – San Francisco (sans-serif typeface)
After [many] comments & further research…
Actually, I think you might be out of luck – that font is not SF Compact & is not the watchOS font. You only see it on early “photos” or possibly mockups of the Watch v1.
If it is an Apple font, it may never have left the building.